Vibratory material handling of large loads requires the use of long strokes applied at a low rate or frequency. These frequencies are significantly lower than those capable of delivery by conventional electromagnet gap motors. The prior art has therefore tended to concentrate on use of mechanical exciters utilizing eccentric weight(s) secured to a shaft rotated by an electric motor. The bearings rotatably mounting the shaft, and the motor armature when directly coupled to this shaft, are subjected to high loads resulting from the oscillating forces created by rotation of the eccentric weight(s), which loads adversely affect the service life of the bearings. In addition, the environment in which the vibratory mechanism must operate is often dusty, or otherwise not conducive to long bearing life, resulting in a further diminution thereof which aggravates the service problems relating to these bearings. Because of the heat generated by the motor, and the fact that its output motion is rotational, shrouding of the motor is difficult and/or very costly. Consequently, the use of such mechanical exciters is generally restricted to environments where exposed electric motors do not represent a hazard.
Electromagnetic vibrators, which do not employ a rotating mass, but instead reciprocate a mass in a straight line motion, have no bearings, obviating the problems associated therewith, and can be completely enclosed or shrouded. However, such vibrators require a controller to supply electric power to an electromagnetic, or preferrably dual opposing magnets which are alternately energized. In order to maximize available stroke amplitudes for the reciprocating mass, static forces acting thereon must be eliminated to the extent possible. This can be achieved by both energizing the electromagnets for an electric conduction period which is of equal time duration and magnitude for each magnet, and synchronizing the magnets to turn on in exact opposite phase to each other.
The present invention provides a controller for an electromagnetic exciter, which controller achieves the above, and which derives both its power and timing from a single phase AC power line and drives each of the dual magnet coils at a frequency which is exactly 1/3 of the power line frequency, with each magnet energized only once per mechanical cycle. While either of the major types of thyristors may be used to drive the magnet coils, Silicon Controlled Rectifiers (SCR) are preferred to triacs on a cost and performance basis, even though the SCR's require a timing train locked into not only the power line subharmonic frequency, but also its polarity. The invention utilizes a division arrangement which allows a variable delay to be inserted into the timing train that permits a wide range of control for the power load applied to the magnets. The invention precludes any imbalance from appearing in the power line due to unequal delays since a single shared device generates the delay for both of the magnet coils. Photon coupled isolators are utilized to prevent noise or other unwanted signals from being introduced into the control circuit.